This drives me crazy! How should I handle it?

Ok, so this happens ALL THE TIME, and it makes me nuts.I hate when I am at a store with my kids (ages 9,8 & 2) and they are grabbing at the candy by the register while I'm saying "No, absolutely not. No candy!" Then a minute later some "kind" stranger comes over all smiles and hands my kids the candy they so generously bought for my children! The reason I didn't get the candy was not because I couldn't afford it, it's because I don't want them to have candy all the time.

I fight with them to eat healthy, fight with them to brush their teeth, 9 out 10 times they were monsters in the store for me and don't need a reward at the end of the trip, my older two are hyper enough without adding sugar, the two year old is SOOO two, giving her sugar is asking for trouble. The point is there are a list of reasons I didn't buy the candy and these well meaning strangers are making my life harder. How should I handle this when it happens, because like an asshole I just smile and say in my fakest voice "oh, say thank you, wasn't that sweet? Haha." I blame it on being raised to always be polite and show gratitude, I know these people are trying to do something nice, but it makes me want to bang my head against the wall. I have literally said to my kids when they've asked "Mom do you have a dollar for this candy?" "Yes, I have a dollar, but I am not spending it on candy, you are not allowed to have that." Yet, people still go and buy them candy! I'm so grateful for kind people, and for anyone that goes out of their way to brighten someone's day, but how do I respectfully decline the treats?